Conventional robotic lawnmowers usually operate within a work area in a garden by traversing the work area according to an irregular movement pattern. Such irregular movements will reduce the risk of tracks being formed in the lawn by the lawnmower as well as the risk of incomplete or unbalanced grass cutting attendance to the entire work area over time. For one common type of robotic lawnmowers, a boundary wire, or guide cable, is used to define the perimeter of the work area. By means of sensors in the robotic lawnmower, the lawnmower will detect when it approaches or crosses the boundary wire, and automatically change its propulsion direction so that the work area is not escaped and the lawnmower remains within the intended work area.
A garden may however be a challenging work area for a robotic lawnmower. A garden is generally of a small scale (under or around tens of meters) and may contain many kinds of obstacles such as, for instance, fountains, trees, bushes, ponds, deck chairs, benches, garden umbrellas (sunshades), statues, rocks, sheds, etc. The small scale and the presence of obstacles may make it hard for the robotic lawnmower to access all parts of the work area by its irregular movement pattern; difficult parts may for instance be where access is partially blocked by one or more obstacles, and/or by an obstacle near a boundary of the work area. The typical work area for a robotic lawnmower is thus heterogeneous and compact in nature, and may result in an insufficient grass-cutting work performance by the robotic lawnmower in the garden.
Some contemporary robotic lawnmowers employ satellite navigation to enhance the robotic lawnmower's ability for accurate navigation within the work area. As is well known per se, a satellite navigation or sat nav system is a system of satellites that provide autonomous geo-spatial positioning with global coverage. It allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few metres, or even centimetres, using signals transmitted by radio from satellites. Receivers process the precise time as well as position and some systems also carrier phase, which can be used as a reference for scientific experiments. A satellite navigation system with global coverage may be termed a global navigation satellite system or GNSS (Global Navigation Satellite System).
Particularly good accuracy of GNSS systems is obtained by using one or more fixed, land-based reference receivers, or beacons, in addition to the satellites. This is called differential GNSS. There are several DGNSS techniques, such as the classical DGNSS (or DGPS), the Real Time Kinematics (RTK) and the Wide Area RTK (WARTK).
The use of GNSS systems requires good reception of satellite signals to work reliably. For a robotic lawnmower which employs satellite navigation, the heterogeneous and compact nature of the typical work area (i.e., the lawn in a garden) will pose an additional challenge. If the satellite signals become blocked by buildings, roofs, sheds, trees, garden umbrellas, awnings, foliage, etc, in or near the work area, the robotic lawnmower may lose track and be momentarily prevented from continuing with satellite navigation-based operation in the work area. For differential GNSS, the signal from a reference receiver or beacon may likewise be blocked by, for example, any of the obstacles mentioned above and/or by radio disturbances such as interference, multi-path propagation delay, etc.
If, because of a momentary situation as described above, the robotic lawnmower loses track of the GNSS signals and is therefore momentarily prevented from continuing with satellite navigation-based operation in the work area, several problems may arise.
First, if the robotic lawnmower has no other navigation means, it will have to suspend its current propulsion/movement and wait until the satellite navigation receiver again receives the satellite signals sufficiently well to recalibrate its current position and resume its movement and satellite navigation-based operation in the work area. It must be recalled that a robotic lawnmower is a device which operates autonomously without the direct assistance or control by a human user.
However, every time a robotic lawnmower is forced to repeatedly suspend its operation and stop its movement momentarily and then resume the operation and movement, the electrical motor and mechanical drive line will be subjected to a certain stress. If such stops occur frequently, which might be the case in a heterogeneous and compact garden, the accumulated wear and tear of critical parts of the robotic lawnmower will be considerable, and a premature breakdown of any of these critical parts is at risk.
Moreover, repeated stops and resumptions of the robotic lawnmower movement will consume a small but not negligible additional amount of electric power, hence causing a penalty in battery power consumption and reducing the maximum operational duration until next recharging of the battery.
Should the robotic lawnmower only stop and keep cutting the grass, this will lead to increased wear of the lawn and will lead to tracks or spots being formed leaving the lawn unevenly cut.
In addition, a repeated habit of intermittent stopping and restarting by the lawnmower may be interpreted by the garden owner as a potential malfunction. As a result, the garden owner may arrive at an ill-founded decision to take the lawnmower out of operation and seek professional service assistance or raise a warranty claim against the seller of the lawnmower. It goes without saying that a lawnmower which is taken out of operation will leave the lawn unattended to and thus provide insufficient grass-cutting work performance.
Furthermore, especially when using lowcost GPS systems (such as single channel DGPS/RTK for example) the time to again acquire a fix on the satellite(s) can take a long time (up to around 20 minutes). This will waste operating time when the robotic work tool is possibly standing still trying to acquire a fix on the satellites. It also reduces the trust that a user has in this system as he will often see the robotic work tool simply stop for no apparent reason.
Furthermore, the loss of a fixation of the satellite(s) also mean that the system is vulnerable to interference and there is a great risk of determining erroneous positions due to so called position jumps.
In view of the problems and shortcomings indicated above, there is a need for an improved manner of operation for a robotic lawnmower in a heterogeneous and compact work area.
The same or similar problems also exist for other robotic work tools. There is thus a need for an improved manner of operation for a robotic work tool in a heterogeneous and compact work area.
In fact, the same or similar problems also exist for other guided vehicles, such as autonomous farming equipment for example. There is thus a need for an improved manner of operation for a guided vehicle.